Compressional Wave Velocity of Fault Damage Zones Constrained by Leaky Modes: Application to the 2019 Ridgecrest Aftershock Sequence

Abstract

Damage zones near a fault are formed by multiple seismic ruptures. They are important structures that influence the nucleation and dynamic rupture of earthquakes. There have been a few methods focusing on imaging damage zone structures, and most of them only obtain shear velocity (Vs) reduction. The leaky mode is a special type of wave propagation in low‐velocity zones, whose energy leaks into high‐velocity layers, contrasting with trapped waves. Leaky mode and trapped S‐waves are separated in time and dominate different components, with leaky mode being more sensitive to compression wave (Vp) structures. We propose applying the leaky mode to constrain the P‐wave velocity structure of a damage zone. In this study, we develop the theory of how to use the characteristic frequency of leaky modes to constrain the Vp parameters in a simple sandwiched low‐velocity structure. First, we discuss the acoustic approximation of leaky modes, demonstrating that it is reasonable to predict the characteristic frequency interval of leaky modes for damage zones with high Vp/Vs ratios. Secondly, we discuss the development of leaky modes for different propagation distances and Vp/Vs ratios and extract the characteristic frequency of leaky modes from simulated waveforms. We demonstrate that the characteristic frequency interval of leaky modes is close to the acoustic solution and is more sensitive to Vp reduction. Thirdly, we selected 73 events from the Ridgecrest aftershock sequence on the southeast side of the 2019 Ridgecrest earthquake rupture zone, where well‐observed leaky modes were present, and extracted three characteristic frequencies (7.64, 10.47, and 14.59 Hz). Finally, we constrain the relationship between Vp reduction and damage zone width, indicating a 14%–29% P‐wave velocity reduction under the B4 array.